The federal budget watchdog says public servants take an average of 11.5 paid sick days a year, compared with 18 days reported by Treasury Board President Tony Clement.
The parliamentary budget office says in a new report that Clement's number includes time missed due to workplace injuries and unpaid sick leave.
It estimates the salary paid for sick days amounted to $871 million in 2011-12, about 68 per cent higher than the estimate 10 years prior, which includes inflation.
But it says growth in the size of the public service, wages and number of paid sick days available all contributed to the doubling of sick-leave costs in the last decade and the use of sick leave between departments varies significantly.
Government numbers 'inflated' by disability leave
The report comes as Clement prepares for a battle with public-sector unions in his quest for budget savings.
The governing Conservatives, who are tabling a budget next week, are taking aim at public service salaries, pensions and sick leave in their efforts to balance the books in 2015.
The PBO report notes that data collected by government departments and provided to them by the Treasury Board doesn't differentiate between sick days taken for regular illness and those taken as part of disability leave.
They do this because workers must exhaust all banked sick days before becoming eligible for disability benefits, it said.
"Consequently, the number of sick days reported by the government is to some extent inflated by those on disability leave," it said.
The data provided by the Treasury Board also "does not enable analysis to determine how significantly sick days have been over-stated due to the inclusion of disability leave," the report said.
More women, unionized and older employees
It also says that direct comparisons with the private sector are difficult.
The average civil servant is older than the average private-sector worker; there are also more women and more unionized employees.
Accounting for these differences, a Statistics Canada study found that public service absenteeism was still 1.1 days higher than in the private sector.